1. Field of the Invention
The present invention relates to an anti-theft tag which when attached to an article notifies a security system if the article is taken without permission. More specifically, the present invention is related to an improved tag which will properly notify the security system even if the tag is attached to an article having a conductive surface.
2. Discussion of the Background
A background burglar-proof tag is disclosed in Japanese Patent Laid-Open No. 8-185,584 in which a resonance circuit of the tag attached to a theft-monitored article resonates radio waves of a predetermined frequency from a radio wave transmitter. In addition, the anti-theft tag includes a detachment-detecting mechanism which detects whether or not the tag is detached from the theft-monitored article, and a detachment-informing section which controls an audible output device based on the output detected by the detachment-detecting device. The resonance circuit of the background anti-theft tag is made of conductive metal foils which have a predetermined shape formed by etching or the like on both faces of an insulating dielectric thin film. For example, a spiral induction circuit of a conductive metal foil having a right-side planar pattern of a capacitor circuit connected to the center of the spiral induction circuit is formed on the thin film surface. The detachment-detecting device has a protruding operation bar which is placed on the attaching face of the tag and acts as a detecting switch which detects if the tag has been detached from the article. The detecting switch is electrically connected to a power source and a buzzer. The detachment-informing section includes a circuit composed of the detecting switch, the power source and the buzzer. When the tag is attached to an article, the operation bar is forced into the detachment-detecting device by way of the article and the detecting switch is turned off. When the tag is detached from the article, the operation bar protrudes and the detecting switch is turned on.
An interrogator antenna and a transponder antenna are placed apart from each other at a predetermined distance at a gateway of a store. In addition, the interrogator and transponder antennas are electrically connected to a control section. The control section transmits radio waves at a given frequency via the interrogator antenna which resonate in the resonance circuit of the anti-theft tag. Further, the control section continuously monitors received signal levels via the transponder antenna. Further the control section connects to a speaker for generating an alarm.
When an article with the anti-theft tag passes between the interrogator and transponder antennas, radio waves from the interrogator antenna resonate in the resonance circuit of the tag attached to the theft-monitored article and the transponder antenna receives a signal with a modulated level. The control section then generates an alarm using the speaker to provide notice that the article has been taken without permission. When the tag is detached from the article, the operation bar protrudes, the detecting switch is turned on, and the buzzer sounds. Thus, theft can be surely monitored.
However, when such a background tag is attached to an article which has a surface made with a conductive material (e.g., aluminum or a ferromagnetic material), the self-inductance of the resonance circuit changes, resulting in a different resonance frequency for an article having a surface formed with an insulating or nonmagnetic material. Thus, such an anti-theft tag is not suitable for articles having conductive or ferromagnetic materials.